426 DB. B. C. A. WINDLE ON DOUBLE [May 21, 



amongst those in my collection (a, fig. 6). On the left side of the 

 right lish, which is itself well-formed and normal in every way, and 

 at about the site of the pectoral fins, there is a pointed projection, 

 representing a second embryo. This projection is unprovided 

 with branchia and has no mouth, but on its under surface there is 

 a single median round patch of pigment, which, from its identity 

 of appearance with certain other conditions yet to be mentioned, I 

 take to be an ill-developed eye. (h) In this case the second fish, 

 though much smaller than its normal fellow, is recognizable as a 

 fish. It possesses a mouth and a normally formed right eye, that 

 of the left side being represented by a circular patch of pigment 

 like that alluded to in the first case, (c) In this case the head of the 

 parasite has a mouth and is of a shape approximating to the normal, 

 but possesses no eyes nor even pigme)it patches to represent them. 



Lereboullet mentions an interesting case (series ii. no. 19), in 

 which he was able to observe two stages in the development of a 

 parasite. He says, " Je mis a part un ceuf (de brochet) age de 

 cinq jours, offrant une large bandelette embryonnaire normale avec 

 son sillon ; mais sur le cote droit de cette bandelette et tout pres 

 de sa base, on voyait se detacher du bourrelet blastodermique un 

 tres-petit tubercule, de forme triangulaire. La presence de ce 

 germe accessoire partant du bourrelet blastodermique m'annon^ait 

 la production d'un embryon double, ou plutot en raison de la 

 petitesse du tubercule, un embryou muni d'une languette analogue 

 a celles que j'avais vues precedemment. Cependant je ne revis cet 

 ceuf que huit jours plus tard. Le Poisson age de treize jours etait 

 eclos et tres-agile. II paraissait simple et regulierement conforme ; 

 mais en I'examinant avec attention, je vis qu'il existait, au niveau 

 de la nageoire pectorale du cote droit, un tubercule a peine sen- 

 sible." Prom this observation it would appear that minor evidences 

 of duplicity may be of more frequent occurrence than would be 

 supposed, but so slight in their nature as to be easily overlooked. 



Before leaving the subject of the various classes into M'hich 

 these double forms can be divided it may be well to note (two 

 points. In the first place, it should be observed that as yet no 

 such form as that known to teratologists as katadidymus — a form, 

 that is, in which two bodies are connected with a single cephalic 

 extremity — has ever been described : a somewhat remarkable fact. 

 And, lastly, mention should be made of a singular and most 

 anomalous form described by Klaussner, in which two embryos 

 quite divided from one another lay upon the same yolk-sac side by 

 side, but with the cephalic end of one by the caudal end of the 

 other, I have not seen or found in any of the papers to which I 

 have referred any parallel to this case, which is difficult to account 

 for by any of the theories of duplicity now holding the field. 



III. General Observations. 



1. Relative positions of the two members. The commonest 

 position, as has been pointed out by Knoch, is side by side, but 



